This invention relates generally to temperature measuring devices or thermistors and, more particularly, to material compositions for high temperature thermistors.
High temperature thermistors are often employed to measure the temperature in a hot water supply, the temperature of a heating furnace, the exhaust gas temperature of a heating furnace, or the exhaust gas temperature of a motor vehicle. In these applications, temperatures may range from about 100 degree Celsius to around 1000 degree Celsius. However, the performance of these thermistors generally degrades at higher temperatures.
In certain applications, the accuracy of these thermistors is particularly important for the overall operation of a system. For example, the performance of engines, catalytic converters, and other devices may depend largely on the accuracy of temperature measurements. In combustion engines, the fuel-air mixture, pressure, temperature, and other factors play an important role in the control of exhaust emissions, such as nitrous oxide (NOx), carbon monoxide (CO), and hydrocarbons (HCs). In addition, modern vehicles are often equipped with catalytic converters, which are disposed in an exhaust line coupled to the engine. These catalytic converters use agents or catalysts, such as reduction and oxidation catalysts, to convert emissions of environmentally unfriendly gases into more environmentally friendly gases. For example, a particular catalytic converter may convert nitrous oxide, carbon monoxide, and hydrocarbons into other forms, such as carbon dioxide and nitrogen. Again, accurate temperature measurements play an important role in the operation of these catalytic converters and the overall combustion engine.
A typical catalytic converter operates at temperatures well above 200 degrees Celsius. The performance of the catalytic converter can be monitored by the temperature differential across the converter. In certain applications, the operating temperature of a catalytic converter may be in the range of 300–600 degrees Celsius and the temperature differential across the catalytic converter may be on the order of 10 degree Celsius. Under certain conditions, the temperature endured by a catalytic converter can reach or exceed 1000 degrees Celsius, e.g., if unburned fuel is present in the exhaust line. Hence, thermistors should preferably withstand high temperatures to ensure reliable operation.
Existing thermistors include an unglazed thermistor, which has chromium (Cr) based oxides that have a negative temperature coefficient (NTC) of resistance. Unfortunately, the resistance values of these Cr-based oxides shift upon sufficient aging, e.g., at 1000 degrees Celsius for 1000 hours. This resistance shift is apparently caused by the loss of chromium oxide (Cr2O3) by volatilization over time.
There is a need, therefore, for an improved thermistor, which can reliably perform at high temperatures over time.